1. Field of the Invention
The present invention relates to a device for manufacturing a drawn object from a source of formable material having a chiralic structure.
2. Description of the Prior Art
Such a drawn object may have any section; it may for example be an optical fiber. This is the case which will be considered by way of non limiting example in the following description.
The monomode fibers usually manufactured for telecommunications always have a little linear birefringence and circular birefringence. Consequently, these fibers keep neither linear polarization nor circular polarization.
It is possible to make the fiber very linear birefringent by breaking up the circular symmetry to the benefit of a planar symmetry.
It is also possible to consider a reverse method, which consists in introducing a high circular birefringence so as to keep the circular polarization.
One solution for creating this circular polarization consists in subjecting the glass fiber to a static twisting stress, for example applied externally by twisting between its two ends: one effect of twisting this fiber is to introduce a circular birefringence therein.
The present invention relates to a manufacturing device for freezing a twisted state induced by a transitorily applied stress. It allows a helical structure or chiralic structure of this fiber to be memorized.
For manufacturing an optical fiber, the most usual glass-making techniques consist in melting the starting materials, fining down and drawing them into fibers by drawing out the molten material. But rather than starting with a molten state, it is very often preferred to start with a preform which prefigures substantially the geometry of the desired fiber which it is then sufficient to draw out.
Thus, in the system of doped silica glass fibers, a means for manufacturing this fiber already proposed consists in rotating the preform at very high speed in the drawing furnace.
This process presents numerous disadvantages: since the preform is not a perfect cylinder, its rotation in the molten viscous state causes the creation of defects in the fiber. By rotating the preform, because of the centrifugal force, the material in the soft state drawn towards the outside undergoes lateral deformation. In fact, the preform which is a bar of glass from 40 to 100 cm long has a high mass. Furthermore, the precessional effect is also important for it. Thus, the defect spectrum obtained for the fiber is not the desired spectrum.
The processes for manufacturing fibers which do not use preforms, for example the process with double crucible in which, for manufacturing an optical fiber, a double crucible is used in which are placed the sheath glass for the external part and the core glass for the inside, and since these raw materials are molten, fined down and drawn, they cannot benefit from this process consisting in rotating the preform because of the difficulties there would be in rotating the double crucible.
To overcome these drawbacks, it is no longer the preform but the fiber which may be rotated. Thus, a fiber is obtained having qualities of conservation of the circular polarization. For that, one manufacturing process consists in causing the fiber to undergo, during drawing into fibers, twisting followed by hardening which freezes in the structure a part of the twisting stresses thus obtained, which destroys the right-left symmetry of the conventional fiber. Thus a chiralic fiber is created.
But in such a device, the winding means rotate, as well as the fiber during its travel through the different coating means, which is a cause of limitation of this twisting speed of the fiber.